The following meanings for the abbreviations used in this specification apply:
3GPP 3rd generation partnership project
ACK Acknowledgement
BW Bandwidth
CA Carrier Aggregation
CC Component carrier
DL Downlink
eNB Enhanced Node B, name for Node B in LTE
FB Feedback
FDD Frequency Division Duplex
LTE Long term evolution
LTE-A LTE-Advanced
MAC Media Access Control
MIMO Multiple Input Multiple Output
NAK Negative Acknowledgement
PDCCH Physical Downlink Control Channel
PUCCH Physical Uplink Control Channel
PUSCH Physical Uplink Shared Channel
RRC Radio Resource Control
SU-MIMO Single User Multiple Input Multiple Output
TDD Time Division Duplex
UE User equipment
UL Uplink
Embodiments of the present invention relate to LTE-Advanced system, which will be part of 3GPP LTE Rel-10. More specifically, some embodiments are focussed on UL ACK/NAK feedback design for component carrier (CC) aggregation to deal with the issues related to timing uncertainty CC re-configuration period.
In the following, carrier aggregation (CA) in LTE-Advanced is described in some more detail. In particular, 3GPP is currently studying a new system called LTE-Advanced which fulfils the IMT-Advanced requirements set by the ITU-R. Topics include e.g. bandwidth extensions beyond 20 MHz, relays, MIMO (multiple input multiple output) enhancements such as advanced multi-user MIMO (MU-MIMO) and introduction of SU-MIMO in uplink.
The bandwidth extension beyond 20 MHz in LTE-Advanced has been decided to be done via component carrier aggregation (CA), in which several Release 8 compatible carriers are aggregated together to form a larger bandwidth.
FIG. 3 shows an example for carrier aggregation, in which five carriers (carrier 1 to carrier 5), each corresponding to a Release 8 bandwidth (BW), can be aggregated to form the maximum bandwidth for LTE-Advanced (100 MHz).
The idea is that each Rel'8 terminal can receive/transmit on one of the CCs, whereas LTE-Advanced terminals supporting CA can receive/transmit on multiple CCs at the same time, thus having support for large bandwidth. It has been agreed that up to 5 CCs could be aggregated in LTE-Advanced in both FDD and TDD systems (Rel-10). This number can be increased in coming releases.
In the case eNB wants to enable transmission/reception via multiple DL CCs for a certain UE, it needs to configure the corresponding CCs via radio resource control (RRC) signaling (→configured but de-activated component carriers). In addition to RRC-level CC-configuration it has been agreed to support MAC-level CC activation/deactivation mechanism in LTE-Advanced. An UE needs to be prepared to receive PDCCH and PDSCH (and transmit PUSCH) via activated component carriers. For deactivated CC(s), UE is prepared to receive neither PDCCH nor PDSCH.
In the following, uplink acknowledgement/negative acknowledgement (UL ACK/NAK) feedback for carrier aggregation is described.
For carrier aggregation, it has been agreed that:                Independent transport block is sent via each CC        One ACK/NAK for each transport block is supported.        
Based on above agreements, multiple ACK/NAKs will be sent during one UL subframe. Furthermore, in typical configuration there is asymmetry between the number of DL and UL component carrier supported by the UE. This needs to be taken into account in the ACK/NAK feedback design as well. To support UL ACK/NAK feedback for CA, the following methods are considered to be promising:                Channel selection (e.g., as defined in TS 36.213) to support up to 4 bits on PUCCH format 1b.        ACK/NAK multiplexing method to support more than 4 bits on PUCCH format 2/2a/2b or other new structure (e.g. multi-sequence modulation).        
Channel selection is a mature method to support up to 4 bits, which is used to support the asymmetry of the UL/DL subframe configuration in Rel'8 TDD. Hence, it is natural to support channel selection in carrier aggregation case as well (limited feedback (FB), up-to four bits). For the case with more than 4 ACK/NAK bits, PUCCH format 2 or some other physical channel needs to be considered. It is also possible to extend channel selection to support more than 4 bits.
In the case of CA, the number of ACK/NAK bits that need to be sent during one UL subframe depends on the number of configured DL CCs (this signaled via RRC signaling) or activated DL CCs (signaled via MAC signaling). However, it is noted that there will be a time period when the eNB cannot be sure about the existing number of DL CCs configured and/or activated by the UE. Such misalignment may happen due to UE processing delays related to RRC signaling. Another source of misalignment is potential signaling errors (PDCCH, ACK, NAK) related to activation/deactivation commands. It is noted that during such uncertainty period the existing UL ACK/NAK signaling methods will face severe problems because of the ambiguity on the ACK/NAK codebook size between eNB and UE. These problems include severe scheduler restrictions and also a very high probability for higher layer errors.
Therefore new mechanisms are needed to support reliable ACK/NAK feedback in all cases.
For related design to support UL ACK/NAK feedback for CA during uncertainty higher layer signaling period, in the following, the existing mechanism is described:
For channel selection on PUCCH format 1b, the channel selection table depends on the number of configured/activated DL CCs, and UE will select one PUCCH resource and QPSK point for transmission based on ACK/NAK status. However, in case the ambiguity about the number of configured/activated DL CCs exists, eNB will have no idea which channel selection table is adopted at UE side. In such case, eNB can not be sure about the ACK/NAK status based on the detected PUCCH resource and QPSK constellation point.
For ACK/NAK multiplexing on PUCCH format 2, similar ambiguity exists if the number of ACK/NAK feedback depends on the configured or activated DL CCs.
So, existing mechanism could not handle such an issue.